Conventionally, a switching power supply apparatus is widely applied in various electronic apparatuses such as copying machines, printers, facsimiles, AV (Audio-Visual) equipment, liquid crystal televisions, plasma display panels, and communication terminals. Such a switching power supply apparatus switches, at a high frequency, a DC voltage obtained by rectifying/smoothing commercial AC power supply and converts the switched DC voltage to a desired voltage at a high efficiency with the use of a small-size transformer.
The switching power supply apparatus having a typical arrangement includes a PWM (Pulse Width Modulation) switching power supply apparatus. The PWM switching power supply apparatus includes a main switching element that switches a DC voltage to be applied to a primary side of a transformer. In the arrangement of the PWM switching power supply apparatus, a desired output voltage on a secondary side of a transformer is obtained by detecting the output voltage on the secondary side of the transformer with the use of a voltage detection circuit and controlling with a control circuit a switching pulse width of the main switching element in accordance with a result of the detection.
FIG. 7 is a circuit diagram illustrating an arrangement of a conventional typical switching power supply apparatus 130. As illustrated in FIG. 7, the switching power supply apparatus 130 includes an input terminal 105, a fuse 106, a filter circuit 103, a bridge diode 104, an input smoothing capacitor C101, a transformer 108, a main switching element 101, a diode 109, an output smoothing capacitor 110, voltage divider resistors 112 and 113, a comparator circuit 114, a control circuit 102, and an output terminal 111.
An AC voltage inputted into the input terminal 105 from a commercial power source is rectified by the bridge diode 104 and the input smoothing capacitor C101 after noise components of the AC voltage is removed by the filter circuit 103. A DC voltage obtained by this rectification is inputted between a high-level power supply line A and a low-level power supply line B.
A series circuit composed of a primary coil n101 of the transformer 108 and the main switching element 101 is connected between the power supply lines A and B. The main switching element 101 is on/off controlled by the control circuit 102.
When the main switching element 101 is turned on, an exciting energy is stored in the primary coil n101.
When the main switching element 101 is turned off, this exciting energy is induced to a secondary coil n102 of the transformer 108. The exciting energy is, then, outputted via the output terminal 111, after smoothed by the diode 109 and the output smoothing capacitor 110.
The output voltage of the output terminal 111 is divided by the voltage divider resistors 112 and 113 and inputted into the comparator circuit 114. The comparator circuit 114 compares a value of a divided voltage of the output voltage with a reference voltage, and feeds back a result of the comparison to the control circuit 102.
The control circuit 102 controls switching of the main switching element 101 (carries out PWM control) based on the result of the comparison inputted from the comparator circuit 114 so that the output voltage of the switching power supply apparatus 130 becomes constant.
Further, the switching power supply apparatus 130 includes the fuse (overcurrent prevention means) 106 connected between one end of the input terminal 105 and the filter circuit 103. When a current of a predetermined amount or more flows, the fuse 106 blows and disconnects the switching power supply apparatus 130 from the commercial power source.
Japanese Unexamined Patent Publication No. 153529/2003 (Tokukai 2003-153529 (published on May 23, 2003)) discloses a technique to stop a switching power supply circuit by providing the switching power supply circuit with a low voltage detection circuit detecting an output voltage to be applied to a load that is connected to a secondary side of a transformer and shutting off feedback from the secondary side to a primary side of the transformer at the time when the low voltage detection circuit finds that the output voltage is decreased to a predetermined value or less.
Various safety standards are defined with respect to electronics devices. Examples of such safety standards are Standard IEC (International Electrotechnical Commission) that is a worldwide safety standard, Standard UL (Underwriters Laboratories Inc.) (US), Standard CSA (Canadian Standards Association) (Canada), and Standard BS (British Standards Institution) (England). Moreover, Electrical Appliance And Material Safety Law is enforced in Japan since Apr. 1, 2001.
In case of Japan, for example, an open/short test is carried out when electrical appliances and materials are certified to the safety standard. Assuming phenomena such as contact with a foreign material, improper wiring processing, and defect in soldering which possibly occur in electronics devices, the open/short test is arranged to check that an unsafe state such as smoking or firing does not occur in a case where (i) ends or terminals of a component is short-circuited and (ii) one end of a component becomes open.
However, even if an electronics device including the conventional switching power supply apparatus complies with the safety standard in production and shipping stages of the electronics device, the conventional switching power supply apparatus had a following problem. That is, when the input smoothing capacitor C101 deteriorates and an insulating resistance decreases, a leak current flowing in the input smoothing capacitor C101 increases (so-called, half short circuit). This causes overheat of the bridge diode 104 and the filter circuit 103 before the fuse 106 blows due to the increase.
Namely, even if the switching power supply apparatus 130 complies with a standard defined by the safety standard at the time of production and shipment, the input smoothing capacitor C101 may deteriorate and the insulating resistance may decrease in a case where the switching power supply apparatus 130 is used after expiration of a warranty period or under an improper condition. When, consequently, the leak current of the input smoothing capacitor C101 increases, there is a time lug between time when the leak current starts to increase and time when the fuse 106 blows. Accordingly, before the fuse 106 blows, an overcurrent may flow into the bridge diode 104 and the filter circuit 103 provided in a preceding stage with respect to the input smoothing capacitor C101. This may overheat the bridge diode 104 and the filter circuit 103. Further, the fuse 106 does not blow, when a value of the current flowing in the fuse 106 does not reach a level for blowing the fuse 106 after the increase of the leak current of the smoothing capacitor C101. As a result, the bridge diode 104 and the filter circuit 103 may overheat.
According to a technique disclosed in Japanese Unexamined Patent Publication No. 153529/2003 (Tokukai 2003-153529 (published on May 23, 2003)), it takes a long time before the switching power supply circuit stops, after a half short circuit occurs. This may cause an overcurrent to flow in a circuit provided in the preceding stage with respect to the input smoothing capacitor C101, before the switching power supply circuit stops. As a result, the circuit may overheat. Namely, according to the technique disclosed in Japanese Unexamined Patent Publication No. 153529/2003 (Tokukai 2003-153529 (published on May 23, 2003)), it takes a long time before it is detected that the output voltage decreases to a predetermined value or less due to a half short circuit, after the half short circuit occurs. Moreover, it takes a long time before the switching power supply circuit is stopped by shutting off feedback from a secondary side of a transformer to a primary side thereof, after it is detected that the output voltage becomes equal to or less than the predetermined value. As a result, an overcurrent flows into the circuit provided in the preceding stage with respect to the input smoothing capacitor C101 before the switching power supply circuit is stopped, after the half short circuit occurs.